Multi-stage compression refrigeration cycle device

ABSTRACT

A multi-stage compression refrigeration cycle device includes a controller that controls rotational speeds of a low-stage side compression mechanism and a high-stage side compression mechanism, and a physical quantity sensor that detects a physical quantity correlated with a pressure of a low-pressure refrigerant. The controller is configured to increase a rotational speed ratio of the rotational speed of the low-stage side compression mechanism to the rotational speed of the high-stage side compression mechanism as the pressure of the low-pressure refrigerant becomes higher, based on the physical amount detected by the physical quantity sensor.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2015-182172 filed on Sep. 15, 2015, the contents of which are incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present disclosure relates to a multi-stage compression refrigeration cycle device that includes multi-stage compression mechanisms.

BACKGROUND ART

Conventionally, for example, Patent Document 1 discloses a multi-stage compression refrigeration cycle device that includes a low-stage side compression mechanism and a high-stage side compression mechanism. The low-stage side compression mechanism compresses a low-pressure refrigerant to an intermediate-pressure refrigerant and then discharges the compressed intermediate-pressure refrigerant. The high-stage side compression mechanism compresses the intermediate-pressure refrigerant, discharged from the low-stage side compression mechanism, to a high-pressure refrigerant and then discharges the compressed high-pressure refrigerant. In this way, the multi-stage compression refrigeration cycle device is designed to pressurize the refrigerant in multiple stages.

In more detail, the multi-stage compression refrigeration cycle device described in Patent Document 1 is configured as a so-called economizer refrigeration cycle. The economizer refrigeration cycle includes a heat radiator that dissipates heat from the high-pressure refrigerant discharged from the high-stage side compression mechanism and an intermediate-pressure expansion valve that decompresses and expands part of the high-pressure refrigerant flowing out of the heat radiator to the intermediate-pressure refrigerant. The economizer refrigeration cycle guides the intermediate-pressure refrigerant, decompressed by the intermediate-pressure expansion valve, to a suction side of the high-stage side compression mechanism.

In this type of economizer refrigeration cycle, a mixed refrigerant composed of the intermediate-pressure refrigerant decompressed by the intermediate-pressure expansion valve and the intermediate-pressure refrigerant discharged from the low-stage side compression mechanism can be drawn into the high-stage side compression mechanism. Thus, the mixed refrigerant at a lower temperature can be drawn into the high-stage side compression mechanism, compared to a case where only the intermediate-pressure refrigerant discharged from the low-stage side compression mechanism is drawn to the high-stage side compression mechanism. Consequently, the compression efficiency of the high-stage side compression mechanism can be improved.

The two-stage compression refrigeration device, described in Patent Document 1, starts its operation by setting the rotational speed of each of a low-stage side compressor and a high-stage side compressor lower than the maximum rotational speed that exhibits the maximum capacity of the compressor, at start-up of the device. Subsequently, the refrigeration device increases the rotational speed in multiple stages. Thus, the spill of oil from the compressor can be suppressed, thereby preventing the occurrence of breakdown of the refrigeration device due to the shortage of oil.

RELATED ART DOCUMENT Patent Document

[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2012-247154

SUMMARY OF THE INVENTION

Such a two-stage compression refrigeration cycle device is required to cool the inside of a refrigerator, which is a space to be cooled, as quickly as possible at start-up of the device. In particular, in summer when the outside air temperature is high, the refrigeration cycle device is increasingly required to cool the inside of the refrigerator more quickly to shorten a cool-down time.

However, a conventional device is configured to control the rotational speeds of the low-stage side compressor and the high-stage side compressor such that the rotational speed ratio of the rotational speed of the low-stage side compression mechanism to that of the high-stage side compression mechanism is constant. In such a device, when the temperature inside the refrigerator is high, the rotational speed of the high-stage side compressor is limited to less than a predetermined protection control value in order to protect a motor provided in the high-stage side compressor. For this reason, the rotational speeds of the low-stage side compressor and the high-stage side compressor might be difficult to increase sufficiently. Consequently, at start-up of the device, it takes a longer time to cool the inside of the refrigerator.

The device described in Patent Document 1, mentioned above, is configured to start its operation by setting the rotational speed of each of a low-stage side compressor and a high-stage side compressor lower than the maximum rotational speed that exhibits the maximum capacity of the compressor, at start-up of the device, and then to increase the rotational speed in multiple stages. However, this device controls only the rotational speed of each of the low-stage side and high-stage side compressors merely to suppress the spill of oil, but never considers any means to shorten the cool-down time.

The cool-down time can be shortened by upsizing the low-stage side compressor and the high-stage side compressor. However, if each compressor is upsized, the cost of the refrigeration cycle device would be increased, and additionally, a mounting space for the refrigeration cycle device would become larger.

Therefore, it is an object of the present disclosure to shorten the cool-down time at start-up of the refrigeration cycle device without upsizing each compressor.

According to an aspect of the present disclosure, a multi-stage compression refrigeration cycle device, includes: a low-stage side compression mechanism that compresses a low-pressure refrigerant to an intermediate-pressure refrigerant and discharges the compressed intermediate-pressure refrigerant; a high-stage side compression mechanism that compresses the intermediate-pressure refrigerant discharged from the low-stage side compression mechanism to a high-pressure refrigerant and discharges the compressed high-pressure refrigerant; a heat radiator that exchanges heat between the high-pressure refrigerant discharged from the high-stage side compression mechanism and exterior air to dissipate heat from the high-pressure refrigerant; an intermediate-pressure expansion valve that decompresses and expands the high-pressure refrigerant flowing out of the heat radiator to an intermediate-pressure refrigerant and then flows out the intermediate-pressure refrigerant to a suction side of the high-stage side compression mechanism; a low-pressure expansion valve that decompresses and expands the high-pressure refrigerant flowing out of the heat radiator to the low-pressure refrigerant; an evaporator that exchanges heat between the low-pressure refrigerant decompressed and expanded by the low-pressure expansion valve and ventilation air to be blown into a space to be cooled, causing the refrigerant to evaporate, and then to flow out the refrigerant to a suction side of the low-stage side compression mechanism; a controller that controls rotational speeds of the low-stage side compression mechanism and the high-stage side compression mechanism; and a physical quantity sensor that detects a physical quantity correlated with a pressure of the low-pressure refrigerant. The controller is configured to increase a rotational speed ratio of the rotational speed of the low-stage side compression mechanism to the rotational speed of the high-stage side compression mechanism as the pressure of the low-pressure refrigerant becomes higher, based on the physical amount detected by the physical quantity sensor.

In this way, the controller is configured to increase the rotational speed ratio of the rotational speed of the low-stage side compression mechanism to the rotational speed of the high-stage side compression mechanism as the pressure of the low-pressure refrigerant becomes higher, based on a physical quantity detected by the physical quantity sensor. Thus, the refrigerating capacity of the evaporator can be improved by increasing the rotational speed of the low-stage side compression mechanism, even though the rotational speed of the high-stage side compressor is limited. Therefore, the cool-down time at start-up of the refrigeration cycle device can be shortened without upsizing each compressor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an entire configuration diagram of a multi-stage compression refrigeration cycle device according to an embodiment;

FIG. 2 is a flowchart of control processing performed by a controller in the multi-stage compression refrigeration cycle device according to the embodiment;

FIG. 3 is a diagram representing the relationship between an optimal rotational speed ratio of the low-stage side compressor to the high-stage side compressor and the pressure of a low-pressure refrigerant;

FIG. 4 is a diagram showing the relationship regarding the time characteristics of the rotational speed ratio of the low-stage side compression mechanism to the high-stage side compression mechanism after the start of a cool-down operation;

FIG. 5 is a diagram showing the relationship between the temperature inside the refrigerator and the cool-down time; and

FIG. 6 is a diagram showing the theoretically determined result of the relationship between the pressure of a low-pressure refrigerant and an optimal intermediate pressure ratio.

DESCRIPTION OF EMBODIMENTS First Embodiment

A first embodiment will be described below with reference to FIGS. 1 to 3. FIG. 1 is an entire configuration diagram of a multi-stage compression refrigeration cycle device in the present embodiment. The multi-stage compression refrigeration cycle device is applied to a refrigerator and serves to cool ventilation air to be blown into the inside of the refrigerator as a space to be cooled, to an ultralow temperature of approximately −30° C. to −10° C.

As illustrated in FIG. 1, the multi-stage compression refrigeration cycle device includes two compressors, namely, a high-stage side compressor 11 and a low-stage side compressor 12. The multi-stage compression refrigeration cycle device is configured to pressurize the refrigerant circulating through the cycle in multiple stages. As the refrigerant, a normal fluorocarbon refrigerant (for example, R404A) can be adopted. The refrigerant contains therein a refrigerant oil (i.e., oil) for lubricating sliding parts of the low-stage side compressor 12 and the high-stage side compressor 11, and at least part of the refrigerant oil circulates through the cycle together with the refrigerant.

The low-stage side compressor 12 is an electric compressor that includes a low-stage side compression mechanism 12 a and a low-stage side electric motor 12 b. The low-stage side compression mechanism 12 a compresses a low-pressure refrigerant to an intermediate-pressure refrigerant and discharges the intermediate-pressure refrigerant. The low-stage side electric motor 12 b rotatably drives the low-stage side compression mechanism 12 a.

The low-stage side electric motor 12 b is an AC motor that has its operation (i.e., the rotational speed) controlled by AC current output from a low-stage side inverter 22. The low-stage side inverter 22 outputs the AC current at a frequency in response to a control signal, output from a refrigerator controller 20 to be described later. The refrigerant discharge capacity of the low-stage side compression mechanism 12 a is changed by controlling the frequency.

Thus, in the present embodiment, the low-stage side electric motor 12 b configures a discharge-capacity changing portion for the low-stage side compressor 12. It is apparent that the low-stage side electric motor 12 b may adopt a DC motor, whereby the rotational speed of the motor may be controlled by a control voltage output from the refrigerator controller 20. A discharge port of the low-stage side compression mechanism 12 a is connected to a side of a suction port of the high-stage side compressor 11.

The high-stage side compressor 11 has substantially the same basic structure as the low-stage side compressor 12. Thus, the high-stage side compressor 11 is an electric compressor that includes a high-stage side compression mechanism 11 a and a high-stage side electric motor 11 b. The high-stage side compression mechanism 11 a compresses the intermediate-pressure refrigerant discharged from the low-stage side compressor 12 to a high-pressure refrigerant and discharges the compressed high-pressure refrigerant.

The high-stage side electric motor 11 b has the rotational speed thereof controlled by AC current output from a high-stage side inverter 21. In the present embodiment, a compression ratio of the high-stage side compression mechanism 11 a is substantially the same as a compression ratio of the low-stage side compression mechanism 12 a.

A discharge port of the high-stage side compression mechanism 11 a is connected to a refrigerant inlet side of a heat radiator 13. The heat radiator 13 is a heat-dissipation heat exchanger that exchanges heat between the high-pressure refrigerant discharged from the high-stage side compressor 11 and air outside the refrigerator (i.e., exterior air) blown by a cooling fan 13 a, thereby dissipating heat from the high-pressure refrigerant to cool the refrigerant.

In the present embodiment, the refrigerator controller 20 configures a controller that controls the rotational speeds of the low-stage side compression mechanism 12 a and the high-stage side compression mechanism 11 a. In more detail, the refrigerator controller 20 configures the controller that controls the rotational speed of each of the low-stage side electric motor 12 b for rotating the low-stage side compression mechanism 12 a and the high-stage side electric motor 11 b for rotating the high-stage side compression mechanism 11 a.

The cooling fan 13 a is an electric blower that has its rotational speed controlled by a control voltage output from the refrigerator controller 20. The blowing air volume of the cooling fan is determined according to its rotational speed. The multi-stage compression refrigeration cycle device in the present embodiment configures a subcritical refrigeration cycle in which a high-pressure side refrigerant pressure does not exceed the critical pressure of the refrigerant while using a fluorocarbon refrigerant as the refrigerant, and thereby the heat radiator 13 functions as a condenser that condenses the refrigerant.

A refrigerant outlet of the heat radiator 13 is connected to a branch portion 14 that branches the flow of the refrigerant flowing out of the heat radiator 13. The branch portion 14 has a three-way joint structure with three flow inlet/outlets. One of the flow inlet/outlets is a refrigerant flow inlet, while the other two are refrigerant flow outlets. Such a branch portion 14 may be formed by joining pipes or by providing a plurality of refrigerant passages in a metal block or a resin block.

One refrigerant outlet of the branch portion 14 is connected to an inlet side of an intermediate-pressure expansion valve 15, while the other refrigerant outlet of the branch portion 14 is connected to an inlet side of a high-pressure refrigerant flow passage 16 a in an intermediate heat exchanger 16. The intermediate-pressure expansion valve 15 is a thermal expansion valve that decompresses and expands the high-pressure refrigerant, flowing out of the heat radiator 13, to an intermediate-pressure refrigerant and then flows out the intermediate-pressure refrigerant to a suction side of the high-stage side compression mechanism 11 a.

More specifically, the intermediate-pressure expansion valve 15 has a thermo-sensitive portion disposed on an outlet side of an intermediate-pressure refrigerant flow passage 16 b in the intermediate heat exchanger 16. The thermo-sensitive portion of the intermediate-pressure expansion valve 15 senses a superheat degree of the refrigerant on the outlet side of the intermediate-pressure refrigerant flow passage 16 b based on the temperature and pressure of the refrigerant on the outlet side of the intermediate-pressure refrigerant flow passage 16 b. The intermediate-pressure expansion valve 15 adjusts its valve opening by a mechanical mechanism such that the sensed superheat degree of the refrigerant reaches a predetermined value previously set. The flow rate of the refrigerant from the intermediate-pressure expansion valve 15 is determined according to its valve opening degree. The outlet side of the intermediate-pressure expansion valve 15 is connected to the inlet side of the intermediate-pressure refrigerant flow passage 16 b.

The intermediate heat exchanger 16 exchanges heat between the intermediate-pressure refrigerant decompressed and expanded by the intermediate-pressure expansion valve 15 and then circulating through the intermediate-pressure refrigerant flow passage 16 b and the other high-pressure refrigerant branched by the branch portion 14 and then circulating through the high-pressure refrigerant flow passage 16 a. The high-pressure refrigerant is decompressed, so that its temperature decreases. Thus, in the intermediate heat exchanger 16, the intermediate-pressure refrigerant circulating through the intermediate-pressure refrigerant flow passage 16 b is heated, while the high-pressure refrigerant circulating through the high-pressure refrigerant flow passage 16 a is cooled.

The specific structure of the intermediate heat exchanger 16 adopts a double pipe heat exchanger structure in which an inner pipe forming the intermediate-pressure refrigerant flow passage 16 b is disposed inside an outer pipe forming the high-pressure refrigerant flow passage 16 a. It is apparent that the high-pressure refrigerant flow passage 16 a may be positioned as the inner pipe, and the intermediate-pressure refrigerant flow passage 16 b may be positioned as the outer pipe. Alternatively, the intermediate heat exchanger 16 may adopt a structure in which refrigerant pipes forming the high-pressure refrigerant flow passage 16 a and intermediate-pressure refrigerant flow passage 16 b are bonded to each other to exchange heat therebetween.

The intermediate heat exchanger 16 shown in FIG. 1 adopts a parallel flow type heat exchanger in which the flow direction of the high-pressure refrigerant circulating through the high-pressure refrigerant flow passage 16 a is aligned with the flow direction of the intermediate-pressure refrigerant circulating through the intermediate-pressure refrigerant flow passage 16 b. Obviously, a counterflow type heat exchanger may be adopted in which the flow direction of the high-pressure refrigerant circulating through the high-pressure refrigerant flow passage 16 a is opposite to the flow direction of the intermediate-pressure refrigerant circulating through the intermediate-pressure refrigerant flow passage 16 b.

The outlet side of the intermediate-pressure refrigerant flow passage 16 b in the intermediate heat exchanger 16 is connected to a side of the suction port of the high-stage side compression mechanism 11 a, mentioned above, via a check valve (not shown). Thus, the high-stage side compression mechanism 11 a in the present embodiment draws a mixed refrigerant including the intermediate-pressure refrigerant flowing out of the intermediate-pressure refrigerant flow passage 16 b and the intermediate-pressure refrigerant discharged from the low-stage side compressor 12.

The outlet side of the high-pressure refrigerant flow passage 16 a in the intermediate heat exchanger 16 is connected to the inlet side of a low-pressure expansion valve 17. The low-pressure expansion valve 17 is a thermal expansion valve that decompresses and expands the high-pressure refrigerant, flowing out of the heat radiator 13, to the low-pressure refrigerant. The low-pressure expansion valve 17 has a substantially the same basic structure as that of the intermediate-pressure expansion valve 15.

More specifically, the low-pressure expansion valve 17 has a thermo-sensitive portion disposed on the side of a refrigerant outflow port of an evaporator 18 to be described later. The thermo-sensitive portion of the low-pressure expansion valve 17 senses a superheat degree of the refrigerant on the outlet side of the evaporator 18 based on the temperature and pressure of the refrigerant on the outlet side of the evaporator 18. The low-pressure expansion valve 17 adjusts its valve opening of the refrigerant by a mechanical mechanism such that the sensed superheat degree of the refrigerant reaches a predetermined value previously set. The flow rate of the refrigerant flowing through the low-pressure expansion valve 17 is determined according to its valve opening degree.

The outlet side of the low-pressure expansion valve 17 is connected to the side of the refrigerant flow inlet of the evaporator 18. The evaporator 18 is a heat-absorption heat exchanger that exchanges heat between the low-pressure refrigerant decompressed and expanded by the low-pressure expansion valve 17 and the ventilation air blown to and circulating through the inside of the refrigerator by a blower fan 18 a, thereby evaporating the low-pressure refrigerant to exhibit the heat absorption effect. The blower fan 18 a is an electric blower that has its rotational speed controlled by a control voltage output from the refrigerator controller 20. The blowing air volume of the blower fan 18 a is determined according to its rotational speed.

Further, the refrigerant flow outlet of the evaporator 18 is connected to the side of the suction port of the low-stage side compression mechanism 12 a.

Next, an electric control unit in the present embodiment will be described. The refrigerator controller 20 includes a well-known microcomputer including a CPU and storage circuits, an output circuit for outputting a control signal or a control voltage to various control target devices, an input circuit into which a detection signal from each sensor is input, and a power source circuit. The CPU conducts control processing and arithmetic processing. The storage circuits include an ROM and an RAM, which store programs, data, and the like. The storage circuit is a non-transitory physical storage medium.

The output side of the refrigerator controller 20 is connected to the low-stage side inverter 22, the high-stage side inverter 21, the cooling fan 13 a, the blower fan 18 a, and the like, mentioned above as the control target devices. The refrigerator controller 20 controls the operations of these control target devices.

The refrigerator controller 20 incorporates therein control units for controlling the operations of these control target devices. A component (i.e., hardware and software) of the refrigerator controller 20 that controls the operation of each control target device configures the control unit for each of the control target devices.

In the present embodiment, a first discharge-capacity control unit 20 a is defined as a component (i.e., hardware and software) that controls the operation of the low-stage side inverter 22 to thereby control the refrigerant discharge capacity of the low-stage side compression mechanism 12 a. A second discharge-capacity control unit 20 b is defined as a component (hardware and software) that controls the operation of the high-stage side inverter 21 to thereby control the refrigerant discharge capacity of the high-stage side compression mechanism 11 a.

Thus, the rotational speed of the low-stage side electric motor 12 b and the rotational speed of the high-stage side electric motor 11 b can be independently controlled from each other by the first discharge-capacity control unit 20 a and the second discharge-capacity control unit 20 b, respectively. It is apparent that the first and second discharge-capacity control units 20 a and 20 b may be configured as separate controllers with respect to the refrigerator controller 20.

The input side of the refrigerator controller 20 is connected to an outside-air temperature sensor 23, an in-refrigerator temperature sensor 24, a low-pressure sensor 25, an intermediate-pressure sensor 26, a high-pressure sensor 27, and the like. Detection signals from these sensors are input to the refrigerator controller 20. The outside-air temperature sensor 23 detects an outside air temperature Tam of the air outside the refrigerator (i.e., exterior air) that exchanges heat with the high-pressure refrigerant in the heat radiator 13. The in-refrigerator temperature sensor 24 detects an air temperature Tfr of the ventilation air that exchanges heat with the low-pressure refrigerant in the evaporator 18. The low-pressure sensor 25 detects the pressure of the low-pressure refrigerant having flowed out of the evaporator 18 and drawn into the low-stage side compressor 12. The intermediate-pressure sensor 26 detects the pressure of the intermediate-pressure refrigerant discharged from the low-stage side compressor 12. The high-pressure sensor 27 detects the pressure of the high-pressure refrigerant discharged from the high-stage side compressor 11. The low-pressure sensor 25 is a physical quantity sensor that detects a physical quantity correlated with the pressure of the low-pressure refrigerant.

An operation panel 30 is connected to the input side of the refrigerator controller 20. The operation panel 30 is provided with an actuation/stop switch, a temperature setting switch, and the like. Operation signals of these switches are input to the refrigerator controller 20. The actuation/stop switch is a request signal outputting portion that outputs an actuation request signal or a stop request signal of the refrigerator. The temperature setting switch is a target temperature setting portion for setting a target cooling temperature Tset inside the refrigerator.

Next, the operation of the multi-stage compression refrigeration cycle device with the above-mentioned configuration in the present embodiment will be described with reference to FIG. 2. FIG. 2 is a flowchart showing control processing executed by the refrigerator controller 20.

The control processing is started when the actuation/stop switch on the operation panel 30 is closed (i.e., turned ON) to output the actuation request signal. Note that the respective control steps in the flowchart shown in FIG. 2 configure various function implementing portions included in the refrigerator controller 20.

First, in step S100, the refrigerator controller 20 reads the detection signals detected by the outside-air temperature sensor 23, the in-refrigerator temperature sensor 24, the low-pressure sensor 25, the intermediate-pressure sensor 26, the high-pressure sensor 27, and the like, as well as the operation signal of the temperature setting switch on the operation panel 30.

In next step S102, it is determined whether the refrigeration cycle device is in a cool-down state. That is, it is determined whether or not a cool-down operation is necessary to be performed to quickly cool the inside of the refrigerator as the space to be cooled. In the present embodiment, the refrigerator controller 20 specifies the outside air temperature based on the detection signal from the outside-air temperature sensor 23, while specifying the target cooling temperature inside the refrigerator based on the operation signal from the temperature setting switch. The refrigerator controller 20 determines that the refrigeration cycle device is to be in the cool-down state when a temperature difference between the outside air temperature and the target cooling temperature is equal to or more than a predetermined temperature. Meanwhile, the refrigerator controller 20 determines that the refrigeration cycle device is not to be in the cool-down state when a temperature difference between the outside air temperature and the target cooling temperature is less than the predetermined temperature.

If it is determined that the refrigeration cycle device is to be in the cool-down state as the temperature difference between the outside air temperature and the target cooling temperature is equal to or more than the predetermined temperature, the refrigerator controller 20 specifies an optimal rotational speed ratio in step S104.

The ROM of the refrigerator controller 20 stores a map, as shown in FIG. 3, representing the relationship between an optimal rotational speed ratio of the low-stage side compressor 12 to the high-stage side compressor 11 and the pressure of a low-pressure refrigerant. The rotational speed ratio is defined as the rotational speed ratio of the rotational speed of the low-stage side compression mechanism 12 a to the rotational speed of the high-stage side compression mechanism 11 a. The optimal rotational speed ratio is a rotational speed ratio at which the refrigerating capacity of the evaporator 18 is maximized. As shown in the figure, as the pressure of the low-pressure refrigerant becomes higher, the optimal rotational speed ratio is specified to increase. In the present embodiment, the relationship between the pressure of the low-pressure refrigerant and the optimal rotational speed ratio is determined experimentally and stored in the ROM of the refrigerator controller 20.

The optimal rotational speed ratio is specified with reference to the map shown in FIG. 3. Specifically, the pressure of the low-pressure refrigerant is specified based on the detection signal detected by the low-pressure sensor 25, and then the optimal rotational speed ratio corresponding to the specified pressure of the low-pressure refrigerant is specified with reference to the map shown in FIG. 3.

In an initial state of the cool-down operation, the temperature inside the refrigerator is high, and the pressure of the low-pressure refrigerant is high. Thus, the optimal rotational speed ratio becomes a relatively large value. When the temperature inside the refrigerator is decreased and the pressure of the low-pressure refrigerant is reduced as the time elapses, the optimal rotational speed ratio gradually becomes smaller.

In next step S106, the refrigerator controller 20 specifies the rotational speed of the low-stage side compressor 12 and the rotational speed of the high-stage side compressor 11. When the temperature inside the refrigerator is high, the rotational speed of the high-stage side compressor 11 is limited to less than a protection control value previously determined in order to protect the motor provided in the high-stage side compressor. To this end, first, the rotational speed of the high-stage side compressor 11 is specified to be a value lower by a predetermined rotational speed than the limited value. Then, the rotational speed of the low-stage side compressor 12 is specified based on the rotational speed of the high-stage side compressor 11 and the optimal rotational speed ratio specified in step S104.

In next step S108, the rotational speeds of the low-stage side compressor 12 and the high-stage side compressor 11 are controlled to take the respective rotational speeds specified in step S106. Specifically, the refrigerator controller 20 instructs the low-stage side compressor 12 and the high-stage side compressor 11 to rotate at the respective rotational speeds specified in step S106.

The low-stage side inverter 22 outputs the AC current at a frequency in response to a control signal, output from the refrigerator controller 20. The refrigerant discharge capacity of the low-stage side compression mechanism 12 a included in the low-stage side compressor 12 is changed by controlling the frequency.

The high-stage side inverter 21 outputs the AC current at a frequency in response to a control signal, output from the refrigerator controller 20. The refrigerant discharge capacity of the high-stage side compression mechanism 11 a included in the high-stage side compressor 11 is changed by controlling the frequency.

The rotational speeds of the high-stage side compression mechanism 11 a and the low-stage side compression mechanism 12 a are controlled to achieve the optimal rotational speed ratio. Accordingly, the rotational speed of the low-stage side compressor 12 is specified to become large, as compared to a case where the rotational speed ratio of the rotational speed of the low-stage side compression mechanism 12 a to the rotational speed of the high-stage side compression mechanism 11 a is constant. Consequently, the refrigerating capacity of the evaporator 18 is maximized.

In next step S110, the refrigerator controller 20 determines whether or not the operation of a refrigeration cycle device 10 is to be stopped. Specifically, whether or not the operation of the refrigeration cycle device 10 is to be stopped is determined based on whether or not a stop request signal is output from the operation panel 30.

If the stop request signal is not output, the determination in step S110 is NO, and then the processing returns to step S100. Subsequently, if the determination in step S102 is YES, the processing of steps S104 to S110 is performed again.

When a temperature difference between the outside air temperature and the target cooling temperature is less than the predetermined temperature, it is determined that the refrigeration cycle device is not to be in the cool-down state, and then the processing proceeds to step S200 to transfer to the normal control. In the normal control, the rotational speeds of the low-stage side compressor and the high-stage side compressor are controlled such that the rotational speed ratio of the rotational speed of the low-stage side compression mechanism 12 a to that of the high-stage side compression mechanism 11 a is constant.

When the actuation/stop switch on the operation panel 30 is open (i.e., turned OFF) to output the stop request signal, the control processing is finished.

FIG. 4 shows the time characteristic of the rotational speed ratio of the low-stage side compression mechanism 12 a to the high-stage side compression mechanism 11 a after the start of the cool-down operation. Referring to the figure, the rotational speed ratio of the low-stage side compression mechanism 12 a to the high-stage side compression mechanism 11 a in the multi-stage compression refrigeration cycle device of the present embodiment is indicated by the solid line. In a comparative example, the rotational speed ratio of the low-stage side compression mechanism 12 a to the high-stage side compression mechanism 11 a, which is set constant, is indicated by the dotted line.

In an initial state of the cool-down operation, the temperature inside the refrigerator is high, and the pressure of the low-pressure refrigerant is high. Thus, the rotational speed ratio of the low-stage side compression mechanism 12 a to the high-stage side compression mechanism 11 a is controlled to become a relatively large value.

When the temperature inside the refrigerator is decreased and the pressure of the low-pressure refrigerant is reduced as the time elapses, the optimal rotational speed ratio gradually becomes smaller. As the time further elapses, the rotational speed ratio of the low-stage side compression mechanism 12 a to the high-stage side compression mechanism 11 a becomes a constant value that is the same as that in the comparative example.

FIG. 5 shows the time characteristic of the temperature inside the refrigerator after the start of the cool-down operation. Referring to the figure, the temperature inside the refrigerator in the multi-stage compression refrigeration cycle device of the present embodiment is indicated by the solid line. In the comparative example where the rotational speed ratio of the low-stage side compression mechanism 12 a to the high-stage side compression mechanism 11 a is set constant, the temperature inside the refrigerator is indicated by the dotted line.

In the multi-stage compression refrigeration cycle device of the present embodiment, the temperature inside the refrigerator is quickly decreased immediately after the start of the cool-down operation, compared to in the comparative example. As a result, in the multi-stage compression refrigeration cycle device of the present embodiment, the cool-down time taken until the temperature inside the refrigerator reaches the target cooling temperature is shortened significantly, compared to in the comparative example.

As mentioned above, the refrigerator controller 20 is configured to increase the rotational speed ratio of the rotational speed of the low-stage side compression mechanism 12 a to the rotational speed of the high-stage side compression mechanism 11 a as the pressure of the low-pressure refrigerant, specified based on the pressure of the low-pressure refrigerant detected by the low-pressure sensor 25, becomes higher. Thus, the refrigerating capacity of the evaporator 18 can be improved by increasing the rotational speed of the low-stage side compression mechanism, even though the rotational speed of the high-stage side compressor is limited. Therefore, the cool-down time at start-up of the refrigeration cycle device can be shortened without upsizing each compressor.

The refrigerator controller 20 may determine whether or not a cool-down operation is to be performed to quickly cool the space to be cooled based on the temperature of the space to be cooled. The refrigerator controller 20 may increase the rotational speed ratio of the rotational speed of the low-stage side compression mechanism to the rotational speed of the high-stage side compression mechanism as the pressure of the low-pressure refrigerant becomes higher, when it is determined that the cool-down operation is to be performed. In this way, the space to be cooled can be quickly cooled by increasing the rotational speed ratio of the rotational speed of the low-stage side compression mechanism to the rotational speed of the high-stage side compression mechanism as the pressure of the low-pressure refrigerant becomes higher, when it is determined that the cool-down operation is to be performed.

The refrigeration cycle device 10 includes the high-pressure sensor 27 that detects the pressure of a high-pressure refrigerant. The refrigerator controller 20 can determine that the cool-down operation is performed when the pressure of the high-pressure refrigerant detected by the high-pressure sensor 27 is equal to or higher than a reference value previously determined.

Other Embodiments

-   (1) In the above-mentioned embodiments, the refrigerator controller     20 specifies the optimal rotational speed ratio based on the     experimentally determined relationship between the pressure of a     low-pressure refrigerant and the optimal rotational speed ratio.     Alternatively, the relationship between the pressure of a     low-pressure refrigerant and the optimal rotational speed ratio can     also be specified theoretically. FIG. 6 is a diagram showing the     theoretically determined result of the relationship between the     pressure of a low-pressure refrigerant at which the refrigerating     capacity of the evaporator 18 is maximized and an optimal     intermediate pressure ratio. The intermediate pressure ratio is     represented as the pressure Pm of the intermediate-pressure     refrigerant/√ (the pressure Pd of the high-pressure refrigerant x     the pressure Ps of the low-pressure refrigerant). The ratio of the     rotational speed of the low-stage side compression mechanism 12 a to     the rotational speed of the high-stage side compression mechanism 11     a can be specified to achieve the intermediate pressure ratio shown     in FIG. 6. -   (2) In the refrigerator controller 20 according to the     above-mentioned embodiments, the rotational speed ratio of the     rotational speed of the low-stage side compression mechanism to the     rotational speed of the high-stage side compression mechanism is     increased as the pressure of the low-pressure refrigerant becomes     higher. Alternatively, the refrigerator controller 20 may detect the     temperature inside the refrigerator, correlated with the pressure of     the low-pressure refrigerant, by using the in-refrigerator     temperature sensor 24, and may increase the rotational speed ratio     as the temperature inside the refrigerator detected by the     in-refrigerator temperature sensor 24 becomes higher. In this case,     the in-refrigerator temperature sensor 24 is a physical quantity     sensor that detects a physical quantity correlated with the pressure     of the low-pressure refrigerant.

The refrigerator controller 20 may specify the pressure of the low-pressure refrigerant based on the physical quantity detected by the in-refrigerator temperature sensor 24. The refrigerator controller 20 may increase the rotational speed ratio of the rotational speed of the low-stage side compression mechanism 12 a to the rotational speed of the high-stage side compression mechanism 11 a as the specified pressure of the low-pressure refrigerant becomes higher.

-   (3) In the above-mentioned embodiments, the refrigerator controller     20 specifies the rotational speed ratio, which is the ratio of the     rotational speed of the low-stage side compression mechanism 12 a to     the rotational speed of the high-stage side compression mechanism 11     a, based on the pressure of the low-pressure refrigerant.     Alternatively, the refrigerator controller 20 may specify the     rotational speed ratio, which is the ratio of the rotational speed     of the low-stage side compression mechanism 12 a to the rotational     speed of the high-stage side compression mechanism 11 a, for     example, based on the pressure of the low-pressure refrigerant and     the pressure of the intermediate-pressure refrigerant. The     refrigerator controller 20 may specify the rotational speed ratio,     which is the ratio of the rotational speed of the low-stage side     compression mechanism 12 a to the rotational speed of the high-stage     side compression mechanism 11 a, based on the pressure of the     low-pressure refrigerant, the pressure of the intermediate-pressure     refrigerant, and the pressure of the high-pressure refrigerant. In     this way, not only the pressure of the low-pressure refrigerant, but     also the pressure of the intermediate-pressure refrigerant or     high-pressure refrigerant can be used to specify the optimal     rotational speed ratio with higher accuracy. -   (4) In the above-mentioned embodiments, the refrigerator controller     20 specifies the rotational speed ratio, which is the ratio of the     rotational speed of the low-stage side compression mechanism 12 a to     the rotational speed of the high-stage side compression mechanism 11     a, based on the pressure of the low-pressure refrigerant.     Alternatively, the refrigerator controller 20 may specify the     rotational speed ratio, which is the ratio of the rotational speed     of the low-stage side compression mechanism 12 a to the rotational     speed of the high-stage side compression mechanism 11 a, for     example, based on the temperature of the low-pressure refrigerant     correlated with the pressure of the low-pressure refrigerant. In     this case, the refrigerator controller 20 may detect, for example,     the temperature of a pipe through which the low-pressure refrigerant     flows with a temperature sensor without directly detecting the     temperature of the low-pressure refrigerant. Alternatively, the     refrigerator controller 20 may specify the rotational speed ratio,     which is the ratio of the rotational speed of the low-stage side     compression mechanism 12 a to the rotational speed of the high-stage     side compression mechanism 11 a, based on the temperature of the     low-pressure refrigerant and the temperature of the     intermediate-pressure refrigerant. Alternatively, the refrigerator     controller 20 may specify the rotational speed ratio, which is the     ratio of the rotational speed of the low-stage side compression     mechanism 12 a to the rotational speed of the high-stage side     compression mechanism 11 a, based on the temperature of the     low-pressure refrigerant, the temperature of the     intermediate-pressure refrigerant, and the temperature of the     high-pressure refrigerant. -   (5) In the above-mentioned embodiments, the refrigerator controller     20 specifies the rotational speed ratio, which is the ratio of the     rotational speed of the low-stage side compression mechanism 12 a to     the rotational speed of the high-stage side compression mechanism 11     a, based on the pressure of the low-pressure refrigerant.     Alternatively, the refrigerator controller 20 may specify the     rotational speed ratio, which is the ratio of the rotational speed     of the low-stage side compression mechanism 12 a to the rotational     speed of the high-stage side compression mechanism 11 a, for     example, based on the outside air temperature and the temperature     inside the refrigerator. In this case, when the ROM of the     refrigerator controller 20 stores the map that specifies an optimal     rotational speed corresponding to the outside air temperature and     the temperature inside the refrigerator, the refrigerator controller     20 can specify the rotational speed ratio, which is the ratio of the     rotational speed of the low-stage side compression mechanism 12 a to     the rotational speed of the high-stage side compression mechanism 11     a by using the map. -   (6) In the above-mentioned embodiments, the refrigerator controller     20 determines that the refrigeration cycle device is in the     cool-down state when a temperature difference between the outside     air temperature and the target cooling temperature is equal to or     more than the predetermined temperature. Alternatively, the     refrigerator controller 20 may determine that the refrigeration     cycle device is to be in the cool-down state when a pressure of the     high-pressure refrigerant is equal to or higher than a protection     control value. The refrigerator controller 20 may determine that the     refrigeration cycle device is to be in the cool-down state when a     temperature difference between the outside air temperature and the     target cooling temperature is equal to or more than the     predetermined temperature, and when a pressure of the high-pressure     refrigerant is equal to or higher than the protection control value. -   (7) In the above-mentioned embodiments, the respective features of     the present disclosure are applied to any multi-stage compression     refrigeration cycle that has two-stage compression mechanisms on the     high-stage side and the low-stage side. However, the respective     features of the present disclosure can also be applied to a     multi-stage compression refrigeration cycle device that includes     three or more stages of compression mechanisms. -   (8) In the above-mentioned embodiments, the refrigerator controller     20 may determine whether or not the pressure of the high-pressure     refrigerant exceeds a threshold value. The refrigerator controller     20 may respectively decrease the rotational speed of the high-stage     side compression mechanism 11 a and the rotational speed of the     low-stage side compression mechanism 12 a for protection of the     refrigeration cycle device when the pressure of the high-pressure     refrigerant is determined to exceed the threshold value. -   (9) In the above-mentioned embodiments, a fluorocarbon refrigerant     (for example, R404A) is adopted as the refrigerant. However, the     refrigerant in use is not limited to the fluorocarbon refrigerant     and may be, for example, a refrigerant containing carbon dioxide as     a main component.

The present disclosure is not limited to the above-mentioned embodiments, and various modifications and changes can be made to the embodiments as appropriate. It is obvious that in the above-mentioned respective embodiments, the elements included in the embodiments are not necessarily essential particularly unless otherwise specified to be essential, except when clearly considered to be essential in principle, and the like.

The refrigerator controller 20 executes the processing in step S102, which corresponds to a determination unit. 

1. A multi-stage compression refrigeration cycle device, comprising: a low-stage side compression mechanism that compresses a low-pressure refrigerant to an intermediate-pressure refrigerant and discharges the compressed intermediate-pressure refrigerant; a high-stage side compression mechanism that compresses the intermediate-pressure refrigerant discharged from the low-stage side compression mechanism to a high-pressure refrigerant and discharges the compressed high-pressure refrigerant; a heat radiator that exchanges heat between the high-pressure refrigerant discharged from the high-stage side compression mechanism and exterior air to dissipate heat from the high-pressure refrigerant; an intermediate-pressure expansion valve that decompresses and expands the high-pressure refrigerant flowing out of the heat radiator to an intermediate-pressure refrigerant and then flows out the intermediate-pressure refrigerant to a suction side of the high-stage side compression mechanism; a low-pressure expansion valve that decompresses and expands the high-pressure refrigerant flowing out of the heat radiator to the low-pressure refrigerant; an evaporator that exchanges heat between the low-pressure refrigerant decompressed and expanded by the low-pressure expansion valve and ventilation air to be blown into a space to be cooled, causing the refrigerant to evaporate, and then to flow out the refrigerant to a suction side of the low-stage side compression mechanism; a high-pressure sensor that detects a pressure of the high-pressure refrigerant; a controller that controls rotational speeds of the low-stage side compression mechanism and the high-stage side compression mechanism; and a physical quantity sensor that detects a physical quantity correlated with a pressure of the low-pressure refrigerant, wherein the controller is configured to increase a rotational speed ratio of the rotational speed of the low-stage side compression mechanism to the rotational speed of the high-stage side compression mechanism as the pressure of the low-pressure refrigerant becomes higher, based on the physical amount detected by the physical quantity sensor, and the controller is configured to increase the rotational speed ratio as the pressure of the low-pressure refrigerant becomes higher, when the pressure of the high-pressure refrigerant detected by the high-pressure sensor is equal to or higher than a predetermined reference value.
 2. The multi-stage compression refrigeration cycle device according to claim 1, wherein the physical quantity sensor is an in-refrigerator temperature sensor that detects a temperature of the space to be cooled, and the controller is configured to increase the rotational speed ratio as the temperature of the space to be cooled, detected by the in-refrigerator temperature sensor, becomes higher.
 3. The multi-stage compression refrigeration cycle device according to claim 1, further comprising: a determination unit that determines whether or not a cool-down operation is necessary to quickly cool the space to be cooled, based on a temperature of the space to be cooled, wherein, the controller is configured to increase the rotational speed ratio of the rotational speed of the low-stage side compression mechanism to the rotational speed of the high-stage side compression mechanism as the pressure of the low-pressure refrigerant becomes higher, when the determination unit determines that the cool-down operation is to be performed.
 4. The multi-stage compression refrigeration cycle device according to claim 3, wherein the determination unit determines that the cool-down operation is necessary when the pressure of the high-pressure refrigerant detected by the high-pressure sensor is equal to or higher than a predetermined reference value.
 5. The multi-stage compression refrigeration cycle device according to claim 1, further comprising a middle-pressure expansion valve that decompresses and expands the high-pressure refrigerant flowing out of the heat radiator to an intermediate-pressure refrigerant, and flows out the intermediate-pressure refrigerant to a suction side of the high-stage side compression mechanism. 